1. Field of the Invention
This invention relates to a radiation image read out device, and more particularly to an improvement in a read out device for reading out a radiation image recorded in a stimulable phosphor which has been exposed to imagewise radiation and stores the energy of the radiation. The read out device detects or measures the light emitted from the stimulable phosphor according to the stored energy when the stimulable phosphor is exposed to and stimulated by stimulating rays after the exposure to the imagewise radiation.
2. Description of the Prior Art
When a stimulable phosphor is exposed to a radiation like X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet rays and so forth, a part of the energy of the radiation is stored in the phosphor. Then, when the stimulable phosphor retaining the stored energy of radiation is exposed to stimulating rays, the stimulable phosphor emits light according to the amount of energy stored therein.
A radiographic image recording system using the stimulable phosphor is known. This system is, as disclosed in U.S. Pat. No. 3,859,527, a method of recording a radiographic image finally on a photosensitive material after reading out the radiographic image recorded in a stimulable phosphor by stimulating the phosphor. In more detail, at first a stimulable phosphor is exposed to an imagewise radiation passing through an object like a human body to record a radiation image of the object. Then, the stimulable phosphor is scanned by a laser beam or the like and emits light according to the energy of radiation stored therein. The emitted light is detected by a photodetector, the output of which is used for modulating an image recording laser beam or the like to record the radiation image on a photosensitive material like a photographic film.
In the above-described method, a large semitransparent mirror inclined at 45.degree. with respect to the optical path of the laser beam is located at a substantial distance from the stimulable phosphor plate. The stimulating rays are deflected by a galvanometer and scans the stimulable phosphor plate moving at a constant speed through the semitransparent mirror. The light emitted from the stimulable phosphor plate is reflected sideward by the semi-transparent mirror and collected by a condenser lens to enter a single photodetector. The output signal of the photodetector is sent to an analog processer after being amplified and is processed therethrough.
The light emitted from the stimulable phosphor plate is diffused and has very low intensity. Therefore, it is necessary to make the light receiving angle as large as possible to collect as much light as possible to obtain high light collecting efficiency. If the light collecting efficiency is low, the signal-to-noise ratio (S/N) is low because of the large statistical fluctuation of photons.
However, in the above-described method it is difficult to make the light receiving angle sufficiently large due to the structure of the device for reading out the image used therein, and accordingly the light collecting efficiency is as low as several percent.
Further, since the image signal obtained by converting the light emitted from the stimulable phosphor is very low in amplitude, the image signal is amplified by a pre-amplifier provided in the vicinity of the photodetector. The amplified signal is sent to the analog processer to conduct the log-conversion and gradation conversion necessary for printing on a film. The stimulable phosphor plate, a light scanning device, a photodetector and amplifiers for amplifying the minute output signals from the photodetector are normally provided within a dark box. On the other hand, the analog processer is generally located outside the box for the convenience of operation. With this arrangement, however, the distance between the amplifiers within the dark box and the analog processer becomes long and accordingly there are possibilities of picking up noise along the lines connecting therebetween which is, for example, as long as 2 m or more. Therefore, the S/N ratio is lowered in the above-described arrangement.
Further, in the image signal obtained by the photodetector is included an alternating current component known as quantum noise. Particularly when the amount of the detected photons is small, the quantum noise is not negligible. In order to eliminate the quantum noise the usual procedure is, to cut off the high frequency component by use of a low-pass filter. Though the low-pass filter is effective to eliminate the quantum noise, it is disadvantageous in that the output signal through the low-pass filter at a given moment in time contains a signal component for the previous moment in time and accordingly the degree of distinction between adjacent picture cells is lowered thereby.
As mentioned above, since the radiation image read out device measures a very minute amount of light or photons, it is liable to be subjected to influence of the quantum noise and the external electrical noise. In order to reduce the quantum noise, it is desirable to raise the light collecting efficiency to reduce the rate of the quantum noise or provide an effective electric circuit in the place of the low-pass filter to electrically remove the quantum noise. On the other hand, in order to reduce the electrical noise, it is possible to use a shielded wire. However, even if the shielded wire is used, it is difficult in practice to completely eliminate the electrical noise. Therefore, it is desirable to eliminate the noise by an electrical noise reduction means.